Frameless Window

### What
Give the user possibility to make the app's window frameless.

### Why
This gives a much cleaner look and is eminently less distracting.

### How
This could be implemented fairly quickly by adding 2 lines of code to the app's main entry point, by default it's `main.js`.
To implement a switch button for this option shouldn't take too many lines of code as well.
[Here's](https://www.electronjs.org/docs/latest/tutorial/window-customization) Electron's documentation about this topic.

**Window border present in Simplenote**

**No window border in vscode**

Start with the app's main entry point, mentioned as main.js, and review Electron's window-customization documentation. Determine how the frameless option should be exposed through a switch button. Done means the app window can display without its border and the user can control that option.

Written by the indexing model from the issue text.